Peelable polyester film with self-venting, process for its production and its use

ABSTRACT

Coextruded, biaxially oriented polyester films which have a base layer (B) and have a heatsealable outer layer (A) that can be peeled from APET/CPET and from CPET, where the outer layer (A) includes
     a) from 80 to 98% by weight of polyester and   b) from 2 to 10% by weight of inorganic and/or organic particles with a median diameter d 50  of from 3 to 12 μm,
 
and where
   c) the polyester is composed of from 12 to 89 mol % of units derived from at least one aromatic dicarboxylic acid and of from 11 to 88 mol % of units derived from at least one aliphatic dicarboxylic acid,
 
and
   d) the ratio calculated from the particle size d 50  of the particles and the layer thickness d A  of the outer layer (A) is greater than or equal to 1.5,
 
and the shrinkage of the films, at least in one direction, is more than 5%. The films of the invention are especially suitable, owing to their property of self-venting, as a lid film for APET/CPET or CPET trays.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to its parent application, GermanPatent Application 103 52 431.2, filed Nov. 10, 2003, herebyincorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The invention relates to a coextruded, peelable and biaxially orientedpolyester film having a base layer (B) and at least one outer layer (A)applied to this base layer. The outer layer (A) is heatsealable and haseasy to moderate peelability, especially to PET trays. In addition, thefilm features self-venting. The heatsealable and peelable outer layer(A) comprises polyester based on aromatic and aliphatic acids andaliphatic diols. In addition, the outer layer (A) comprises particles.The invention further relates to a process for producing the film and toits use.

BACKGROUND OF THE INVENTION

For ready-prepared meals, there are currently double-digit growth ratesin Europe. The ready-prepared meals are transferred to trays after theirpreparation (cf. FIG. 1). A film which is heatsealed to the edge of thetray seals the packaging and protects the ready-prepared meal fromexternal influences. The ready-prepared meals are suitable, for example,for heating in a microwave, for heating in a conventional oven or forheating in a microwave and in a conventional oven. In the latter case,the ready-prepared meal and the packaging have to be “dual ovenable”(=suitable for microwave and conventional ovens). As a consequence ofthe temperatures existing in the conventional oven (up to 220° C.),particularly high demands are made on the packaging material (tray andlid film).

Both for the tray and for the lid film, only selected materials can beconsidered for dual ovenable applications. Typical materials for thetrays are in this case CPET (crystalline PET), aluminum, cardboardcoated with PET or with PET film. In the case of CPET trays (cf. FIG. 1a), the thick crystalline and usually pigmented, i.e. particle-filled,CPET film provides the stability of the tray, even at the comparativelyhigh temperatures in the conventional oven. APET/CPET trays (cf. FIG. 1b) include externally a CPET layer and internally, i.e. facing towardthe ready-prepared meal, an APET layer. The thick crystalline andusually pigmented, i.e. particle-filled, CPET layer provides thestability of the tray; in contrast, the amorphous PET essentiallyimproves the adhesion of the film to the tray. PET=polyethyleneterephthalate, APET=amorphous PET, CPET=crystalline PET.

In dual ovenable applications, the material used for the lid film isgenerally PET which is dimensionally stable and solid even at 220° C.Materials such as PP or PE are ruled out from the outset because oftheir low melting points. The requirements on the lid film are bestfulfilled by biaxially oriented polyester film.

When preparing the ready-prepared meal in the oven, the polyester filmis removed by hand from the tray shortly before heating or shortly afterheating. When this is done, the polyester film must on no account startto tear, start and continue to tear or tear off. The removal of the filmfrom the tray without the film starting or continuing to tear or tearingoff is also referred to in the foods industry as peeling. For thisapplication, the polyester film therefore has to be not onlyheatsealable, but in particular also peelable. For a given material andgiven overall thickness of the film, the peelability of the film isdetermined mainly by the properties of the surface layer of the filmwhich is sealed to the tray.

The peelability of films can be determined relatively simply in thelaboratory using a tensile strain tester (for example from Zwick,Germany) (cf. FIG. 2). For this test, two strips of width 15 mm andlength approx. 50 mm are first cut out of the polyester film and thetray and sealed to one another. The sealed strips are, as shown in FIG.2, clamped into the clips of the tester. The “angle” between the filmclamped in the upper clip and the tray strip is 180°. In this test, theclips of the tester are moved apart at a speed of 200 mm/min, and in themost favorable case the film is fully peeled off from the tray (cf., forexample, ASTM-D 3330).

In this test, a distinction is to be drawn between essentially twodifferent mechanisms.

In the first case, the tensile force rises rapidly in the course of thepulling procedure up to a maximum (cf. FIG. 3 a) and then falls directlyback to zero. When the maximum force is attained, the film starts totear or, before delamination from the tray, tears off, which results inthe force falling immediately back to zero. The film is in this case notpeelable, since it is destroyed. The behavior of the film can rather bedescribed as a kind of “welding” to the tray. The destruction of thefilm on removal from the tray is undesired, because this complicates theeasy opening of the packaging without tools such as scissors or knives.

In contrast, a peelable film is obtained when the tensile force or thepeeling force rises up to a certain value (i.e. up to a certain plateau)and then remains approximately constant over the distance over which thetwo strips are sealed together (cf. FIG. 3 b). In this case, the filmdoes not start to tear, but rather can be peeled as desired off the traywith a low force input.

The size of the peeling force is determined primarily by the polymersused in the sealing layer (A) (cf. FIG. 4, polymer 1 and polymer 2). Inaddition, the size of the peeling force is dependent in particular onthe heatsealing temperature employed. The peeling force generally riseswith the heatsealing temperature. With increasing heatsealingtemperature, the risk increases that the sealing layer might lose itspeelability. In other words, a film which is peelable when a lowheatsealing temperature is employed loses this property when asufficiently high heatsealing temperature is employed. This behavior isto be expected in particular in the case of polymers which exhibit thecharacteristics shown in FIG. 4 for polymer 1. This behavior which tendsto generally occur but is rather unfavorable for the application has tobe taken into account when designing the sealing layer. It has to bepossible to heatseal the film in a sufficiently large temperature rangewithout the desired peelability being lost (cf. polymer 2 in FIG. 4). Inpractice, this temperature range is generally from 150 to 220° C.,preferably from 150 to 200° C. and more preferably from 150 to 190° C.

When the ready-prepared meal is heated, water vapor passes from the foodinto the space between food and film. The water vapor has to be ventedduring the heating, since the film or the tray would otherwise burstopen at a point which cannot be determined beforehand. In order toprevent this, the film has to be pierced before heating. However, theconsumer often does not do this or else often simply forgets. It istherefore desirable to provide a polyester film for which it is notnecessary to pierce the film before heating (film with self-venting).

The heatsealable and peelable layer is applied to the polyester film inaccordance with the prior art, generally by means of offline methods(i.e. in an additional process step following the film production). Thismethod initially produces a “standard polyester film” by a customaryprocess. The polyester film produced in this way is then coated offlinein a further processing step in a coating unit with a heatsealable andpeelable layer. In this process, the heatsealable and peelable polymeris initially dissolved in an organic solvent. The final solution is thenapplied to the film by a suitable application process (knifecoater,patterned roller, die). In a downstream drying oven, the solvent isevaporated and the peelable polymer remains on the film as a solidlayer.

Such an offline application of the sealing layer is comparativelyexpensive for several reasons. First, the film has to be coated in aseparate step in a special apparatus. Second, the evaporated solvent hasto be condensed again and recycled, in order thus to minimize pollutionof the environment via the waste air. Third, complicated control isrequired to ensure that the residual solvent content in the coating isvery low.

Moreover, in an economic process, the solvent can never be completelyremoved from the coating during the drying, in particular because thedrying procedure cannot be of unlimited duration. Traces of the solventremaining in the coating subsequently migrate via the film disposed onthe tray into the foods where they can distort the taste or even damagethe health of the consumer.

Various peelable, heatsealable polyester films which have been producedoffline are offered on the market. The polyester films differ in theirstructure and in the composition of the outer layer (A). Depending ontheir (peeling) properties, they have different applications. It iscustomary, for example, to divide the films from the applicationviewpoint into films having easy peelability (easy peel), havingmoderate peelability (medium peel) and having strong, robust peelability(strong peel). The essential quantifiable distinguishing feature betweenthese films is the size of the particular peeling force according toFIG. 3 b. A division is undertaken at this point as follows:

Easy peelability Peeling force in the range (easy peel) from about 1 to4 N per 15 mm of strip width Moderate peelability Peeling force in therange (medium peel) from about 3 to 8 N per 15 mm of strip width Strong,robust peelability Peeling force in the range (strong peel) of more than5 N per 15 mm of strip width

A peelable film which has self-venting on heating is already disclosedby the prior art.

WO 02/26493 describes a polymer film laminate which includes a substratefilm in which a sealable and peelable layer is applied on one side and ashrinkable film is laminated onto the other side. The shrinkable filmhas a shrinkage of from 10 to 80% within a temperature range of from 55to 100° C., the ratio of the shrinkage values in TD to MD of the filmbeing in the range from 1:1 to 10:1. The complicated lamination processprovides the market with a relatively costly solution. In addition, inthe event of excessive shrinkage, or in the event of an excessivelystrong seal seam in conjunction with excessive shrinkage, it can beobserved that the tray warps in the course of heating and is deformedwhen taken from the oven.

In addition, sealable PET films are already known.

EP-A-0 035 835 describes a coextruded sealable polyester film to whichparticles whose average particle size exceeds the layer thickness of thesealing layer are added in the sealing layer to improve the winding andprocessing performance. The polymer of the sealing film layer issubstantially a polyester copolymer which is based on aromaticdicarboxylic acids and also aliphatic diols. The particulate additivesform surface elevations which prevent undesired blocking and adhesion ofthe film to rolls or guides. The selection of particles having adiameter greater than the sealing layer worsens the sealing performanceof the film. No information is given in the document on the sealingtemperature range of the film. The seal seam strength is measured at140° C. and is in the range from 63 to 120 N/m (corresponding to from0.97 to 1.8 N/15 mm of film width). There are no indications in thedocument concerning the peeling performance of the film with respect toAPET/CPET or CPET trays.

EP-A 0 379 190 describes a coextruded, biaxially oriented polyester filmwhich comprises a carrier film layer of polyester and at least onesealing film layer of a polyester composition. The sealing film layermay comprise aliphatic and aromatic dicarboxylic acids and alsoaliphatic diols. The polymer for the sealing film layer comprises twodifferent polyesters A and B, of which at least one (polyester B)contains aliphatic dicarboxylic acids and/or aliphatic diols. Thesealing energy which is measured between two sealing film layers facingeach other and joined together (=fin sealing) is more than 400g_(force)·cm/15 mm (=more than 4 N·cm/15 mm), and the sealing film layermay comprise inorganic and/or organic fine particles which are insolublein the polyester, in which case the fine particles are present in anamount of from 0.1 to 5% by weight, based on the total weight of thesealing film layer. In the examples of EP-A-0 379 190, organicparticles, if they are used at all, are used in maximum amounts of 0.3%by weight. Although the film features good peeling properties (havingplateau character in the peeling diagram, see above) with respect toitself (i.e. sealing film layer with respect to sealing film layer),there is no information about the peeling performance with respect toAPET/CPET and CPET trays. In particular, the film of this invention isin need of improvement in its producibility and its processibility (theraw materials tend to adhere).

WO A-96/19333 describes a process for producing peelable films, in whichthe heatsealable, peelable layer is applied inline to the polyesterfilm. In the process, comparatively small amounts of organic solventsare used. The heatsealable, peelable layer comprises a copolyester whichhas

-   from 40 to 90 mol % of an aromatic dicarboxylic acid,-   from 10 to 60 mol % of an aliphatic dicarboxylic acid,-   from 0.1 to 10 mol % of a dicarboxylic acid containing a free acid    group or a salt thereof,-   from 40 to 90 mol % of a glycol containing from 2 to 12 carbon atoms    and-   from 10 to 60 mol % of a polyalkyldiol.

The coating is applied to the film from an aqueous dispersion or asolution which contains up to 10% by weight of organic solvent. Theprocess is restricted with regard to the polymers which can be used andthe layer thicknesses which can be achieved for the heatsealable,peelable layer. The maximum achievable layer thickness is specified as0.5 μm. The maximum seal seam strength is low, and is from 500 to 600g/25 mm², or [(from 500 to 600)/170] N/15 mm of film width.

WO 02/059186 A1 describes a process for producing peelable films, inwhich the heatsealable, peelable layer is likewise applied inline to thepolyester film. The films may be white or transparent. In this case, theheatsealable, peelable layer is produced by employing melt-coating, andit is preferably the longitudinally stretched film which is coated withthe heatsealable, peelable polymer. The heatsealable, peelable polymercontains polyesters based on aromatic and aliphatic acids, and alsobased on aliphatic diols. The copolymers disclosed in the examples haveglass transition temperatures of below −10° C.; such copolyesters aretoo soft, which is why they cannot be oriented in customary rollstretching methods (adhesion to the rolls). The thickness of theheatsealable, peelable layer is less than 8 μm. In WO 02/059186 A1, themelt-coating known per se is delimited from the extrusion coating knownper se technically and by the viscosity of the melt. A disadvantage ofthe process is that only comparatively fluid polymers (max. 50 Pa·s)having a low molecular weight can be used. This results indisadvantageous peeling properties of the film. Moreover, the coatingrate in this process is limited, which makes the production processuneconomic. With regard to quality, faults are observed in theappearance of the film which are visible, for example, as coatingstreaks. In this process, it is also difficult to obtain a uniformthickness of the sealing layer over the web width of the film, which inturn leads to nonuniform peeling characteristics.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a heatsealable andpeelable, biaxially oriented polyester film which features outstandingpeeling properties, in particular with respect to APET/CPET or CPETtrays. Neither it nor its production process should any longer have thedisadvantages of the prior art films and should in particular have thefollowing features:

-   -   an easy to moderate peelability (easy peel to medium peel) with        respect to CPET and the APET side of APET/CPET trays. The        peeling force should be in the range from 1 to 8 N per 15 mm,        preferably in the range from 2.0 to 8 N per 15 mm and more        preferably in the range from 2.5 to 8 N per 15 mm of film strip        width;    -   no organic solvent residues are present in the heatsealable and        peelable layer;    -   the heatsealable and peelable layer, with respect to CPET trays        and the APET side of APET/CPET trays, has a minimum sealing        temperature of 165° C., preferably 160° C., more preferably 155°        C., and a maximum sealing temperature of generally 220° C.,        preferably 200° C. and more preferably 190° C.;    -   it is produced employing processes in which no organic solvents        are used from the outset;    -   it should have such properties (shrinkage characteristics and        surface topography) that it has self-venting on heating (of        ready-prepared meals) both in a microwave and in a conventional        oven, without the tray deforming;    -   the film can be produced economically. This also means, for        example, that the film can be produced using stretching        processes which are customary in the industry. In addition, it        should be possible to produce the film at machine speeds of up        to 500 m/min which are customary today;    -   a good adhesion (preferably greater than 2 N/15 mm of film        width) between the individual layers of the film is ensured for        their practical employment;    -   in the course of the production of the film, it is guaranteed        that the regrind can be fed back to the extrusion in an amount        of up to approx. 60% by weight, without significantly adversely        affecting the physical (the tensile strain at break of the film        in both directions should not decrease by more than 10%), but in        particular the optical, properties of the film.

In addition, care should be taken that the film can be processed onhigh-speed machines. On the other hand, the known properties whichdistinguish polyester films should at the same time not deteriorate.These include, for example, the good mechanical properties (the modulusof elasticity of the biaxially stretched films in both orientationdirections should be greater than 3000 N/mm², preferably greater than3500 N/mm² and more preferably greater than 4000 N/mm²), the windingperformance and the processibility of the film, in particular in theprinting, laminating or in the coating of the film with metallic orceramic materials.

Heatsealable refers here to the property of a coextruded, multilayerpolyester film which has at least one base layer (B) and has at leastone outer layer (=heatsealable outer layer) which can be bonded by meansof sealing jaws by applying heat (140 to 220° C.) and pressure (2 to 5bar) within a certain time (0.2 to 2 s) to itself (fin sealing), or to asubstrate made of a thermoplastic (=lap sealing, here in particular toCPET and APET/CPET trays), without the carrier layer (=base layer)itself becoming plastic. In order to achieve this, the polymer of thesealing layer generally has a distinctly lower melting point than thepolymer of the base layer. When the polymer used for the base layer is,for example, polyethylene terephthalate having a melting point of 254°C., the melting point of the heatsealable layer is generally less than230° C., in the present case preferably less than 210° C. and morepreferably less than 190° C.

Peelable refers here to the property of the inventive polyester filmwhich comprises at least one layer (=heatsealable and peelable outerlayer (A)), after heatsealing to a substrate, of being able to be pulledfrom the substrate in such a way that the film neither starts to tearnor tears off. The bond of heatsealable film and substrate breaks in theseam between the heatsealed layer and substrate surface when the film isremoved from the substrate (cf. also Ahlhaus, O. E.: Verpackung mitKunststoffen [Packaging with plastics], Carl Hanser Verlag, p. 271,1997, ISBN 3-446-17711-6). When the film heatsealed to a test strip ofthe substrate is removed in a tensile strain testing instrument at apeeling angle of 180° in accordance with FIG. 2, the tensile strainbehavior of the film according to FIG. 3 b is then obtained. Oncommencement of the peeling of the film from the substrate, the forcerequired for this purpose rises, according to FIG. 3 b, up to a certainvalue (e.g. 4 N/15 mm) and then remains approximately constant over theentire peeling operation, but is subject to larger or smaller variations(approx. ±20%)

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of two exemplary sealed trays;

FIG. 2 is a schematic illustration of a tensile strain measuringtechnique;

FIG. 3 a is an exemplary diagram of tensile strain at break for a filmhaving weldable behavior;

FIG. 3 b is an exemplary diagram of tensile strain at break for a filmhaving peelable behavior;

FIG. 4 is an exemplary diagram of tensile strain at break for filmshaving weldable and peelable behavior;

FIG. 5 is an exemplary diagram of the correlation between sealingtemperature and peeling force.

DETAILED DESCRIPTION OF THE INVENTION

The object is achieved by providing a coextruded, biaxially orientedpolyester film which has a base layer (B) and has a heatsealable outerlayer (A) that can be peeled at least from APET/CPET and CPET, where theouter layer (A) comprises

-   a) from 80 to 98% by weight of polyester and-   b) from 2 to 10% by weight of inorganic and/or organic particles    with a median diameter d₅₀ of from 3.0 to 12.0 μm (based in each    case on the mass of the outer layer (A)) and where-   c) the polyester is composed of from 12 to 89 mol % of units derived    from at least one aromatic dicarboxylic acid and of from 11 to 88    mol % of units derived from at least one aliphatic dicarboxylic    acid,    -   where the total of the dicarboxylic-acid-derived molar        percentages is 100, and-   d) the ratio calculated from the particle size d₅₀ and the layer    thickness d_(A) of the outer layer (A) is greater than or equal to    1.5,    and the shrinkage of the film, at least in one direction, is more    than 5%, measured at circulated air temperature 100° C. over a    period of 15 min.

The thickness of the outer layer (A) d_(A) is from 0.7 to 0.8 μm.

The abovementioned parameters are each to be interpreted as preferredvalues.

The material of the outer layer (A) thus includes predominantly apolyester and inorganic and/or organic particles. The polyester iscomposed of units which are derived from aromatic and aliphaticdicarboxylic acids. The units which derive from the aromaticdicarboxylic acids are present in the polyester in an amount ofpreferably from 12 to 89 mol %, in particular from 30 to 84 mol %, morepreferably from 40 to 82 mol %. The units which derive from thealiphatic dicarboxylic acids are present in the polyester in an amountof preferably from 11 to 88 mol %, preferably from 16 to 70 mol %, morepreferably from 18 to 60 mol %, and the molar percentages always add upto 100%. The diol units corresponding thereto likewise always make up100 mol %.

Preferred aliphatic dicarboxylic acids are succinic acid, pimelic acid,suberic acid, azelaic acid, sebacic acid, glutaric acid and adipic acid.Particular preference is given to azelaic acid, sebacic acid and adipicacid.

Preferred aromatic dicarboxylic acids are terephthalic acid, isophthalicacid and 2,6-naphthalenedicarboxylic acid, in particular terephthalicacid and isophthalic acid.

Preferred diols are ethylene glycol, butylene glycol and neopentylglycol.

In general, the polyester comprises the following dicarboxylates andalkylenes, based in each case on the total amount of dicarboxylate ortotal amount of alkylene:

from 12 to 89 mol %, preferably from 25 to 79 mol % and more preferablyfrom 30 to 72 mol %, of terephthalate;

from 0 to 25 mol %, preferably from 5 to 20 mol % and more preferablyfrom 10 to 20 mol %, of isophthalate;

from 11 to 88 mol %, preferably from 16 to 70 mol % and more preferablyfrom 17 to 58 mol %, of azelate;

from 0 to 50 mol %, preferably from 0 to 40 mol % and more preferablyfrom 0.2 to 30 mol %, of sebacate;

from 0 to 50 mol %, preferably from 0 to 40 mol % and more preferablyfrom 0 to 30 mol %, of adipate;

more than 30 mol %, preferably more than 40 mol % and more preferablymore than 50 mol %, of ethylene or butylene.

Up to 10% by weight of the material of the outer layer (A) includesfurther additives, auxiliaries and/or other additives which arecustomarily used in polyester film technology.

In a favorable embodiment, the material of the outer layer (A)additionally contains from 2 to 18% by weight, preferably from 5 to 17%by weight and more preferably from 7 to 16% by weight, of a polymerwhich is incompatible with polyester (anti-PET polymer).

It has been found to be appropriate to produce the main polyester of theouter layer (A) from two separate polyesters I and II which are fed tothe extruder for this layer as a mixture.

The heatsealable and peelable outer layer (A) is distinguished bycharacteristic features. It has a sealing commencement temperature(=minimum sealing temperature) with respect to APET/CPET and CPET traysof not more than 165° C., preferably not more than 160° C. and morepreferably not more than 155° C., and a peeling force strength withrespect to APET/CPET and CPET trays of at least 1.5 N, preferably atleast 2.0 N, more preferably at least 2.5 N (always based on 15 mm filmwidth). The heatsealable and peelable outer layer (A), with respect toAPET/CPET and CPET trays, has a max. sealing temperature of generally220° C., preferably 200° C. and more preferably 190° C., and a filmwhich is peelable with respect to APET/CPET and CPET trays is obtainedwithin the entire sealing range. In other words, this film in the 180°tensile experiment according to FIG. 2 provides a curve according toFIG. 3 b. The term trays can be equated with materials in general.

For the preferred, abovementioned ranges, the peeling results can alsobe described numerically. According to the present experimentalinvestigations, the peeling results can be correlated to one anothersimply by the following relationship between the sealing temperature(T=δ in ° C.) and the peeling force (in N/15 mm)0.02·δ/° C.−0.8≦peeling force F/N per 15 mm≦0.033·δ/° C.+1.4

This relationship is depicted graphically in FIG. 5 for illustration.

According to the invention, the film features self-venting as a resultof increased shrinkage at least in one direction. According to theinvention, the shrinkage in this direction is preferably more than 5%,measured at a circulated air temperature of 100° C. and a time of 15 min(cf. test methods). In the preferred embodiment, the shrinkage in thisdirection is more than 8% and, in the particularly preferred embodiment,the shrinkage in this direction is more than 10%. The preferredshrinkage direction specified is the direction at right angles to thefilm web direction (=TD). In addition, the film may equally have thisshrinkage also in MD (film web direction). When the film shrinks in bothdirections, a maximum shrinkage in both directions of 20% is entirelysufficient for self-venting. In contrast, when the film shrinks only inthe preferred direction, the maximum shrinkage should if possible notexceed 30% (measured at circulated air temperature 100° C. and a time of15 min), since problems can otherwise occur when the film is heatsealedto the tray.

The film of the present invention has a base layer (B) and at least oneinventive outer layer (A). In this case, the film has a two-layerstructure. In a preferred embodiment, the film has a three- or more thanthree-layer structure. In the case of the particularly preferredthree-layer embodiment, it includes the base layer (B), the inventiveouter layer (A) and an outer layer (C) on the opposite side to the outerlayer (A); A-B-C film structure. In a four-layer embodiment, the filmcomprises an intermediate layer (D) between the base layer (B) and theouter layer (A) or (C).

The base layer of the film preferably includes at least 80% by weight ofthermoplastic polyester, based on the weight of the base layer (B).Suitable for this purpose are, for example, polyesters of ethyleneglycol and terephthalic acid (=polyethylene terephthalate, PET), ofethylene glycol and naphthalene-2,6-dicarboxylic acid (=polyethylene2,6-naphthalate, PEN), of 1,4-bishydroxymethylcyclohexane andterephthalic acid (=poly-1,4-cyclohexanedimethylene terephthalate, PCDT)and also of ethylene glycol, naphthalene-2,6-dicarboxylic acid andbiphenyl-4,4′-dicarboxylic acid (=polyethylene 2,6-naphthalatebibenzoate, PENBB). Preference is given to polyesters which containethylene units and include, based on the dicarboxylate units, at least90 mol %, more preferably at least 95 mol %, of terephthalate or2,6-naphthalate units. The remaining monomer units stem from otherdicarboxylic acids or diols. Advantageously, copolymers or mixtures orblends of the homo- and/or copolymers mentioned can also be used for thebase layer (B). In the specification of the amounts of the dicarboxylicacids, the total amount of all dicarboxylic acids is 100 mol %.Similarly, the total amount of all diols also adds up to 100 mol %.

Suitable other aromatic dicarboxylic acids are preferablybenzenedicarboxylic acids, naphthalenedicarboxylic acids (for examplenaphthalene-1,4- or -1,6-dicarboxylic acid), biphenyl-x,x′-dicarboxylicacids (in particular biphenyl-4,4′-dicarboxylic acid),diphenylacetylene-x,x′-dicarboxylic acids (in particulardiphenylacetylene-4,4′-dicarboxylic acid) or stilbene-x,x′-dicarboxylicacids. Of the cycloaliphatic dicarboxylic acids, mention should be madeof cyclohexanedicarboxylic acids (in particularcyclohexane-1,4-dicarboxylic acid). Of the aliphatic dicarboxylic acids,the (C₃–C₁₉)alkanedioic acids are particularly suitable, and the alkanemoiety may be straight-chain or branched.

Suitable other aliphatic diols are, for example, diethylene glycol,triethylene glycol, aliphatic glycols of the general formulaHO—(CH₂)_(n)—OH where n is an integer from 3 to 6 (in particularpropane-1,3-diol, butane-1,4-diol, pentane-1,5-diol and hexane-1,6-diol)or branched aliphatic glycols having up to 6 carbon atoms,cycloaliphatic, optionally heteroatom-containing diols having one ormore rings. Of the cycloaliphatic diols, mention should be made ofcyclohexanediols (in particular cyclohexane-1,4-diol). Suitable otheraromatic diols correspond, for example, to the formula HO—C₆H₄—X—C₆H₄—OHwhere X is —CH₂—, —C(CH₃)₂—, —C(CF₃)₂—, —O—, —S— or —SO₂—. In addition,bisphenols of the formula HO—C₆H₄—C₆H₄—OH are also very suitable.

It is particularly advantageous when a polyester copolymer based onterephthalate and certain amounts (preferably <20 mol %) of isophthalicacid or based on terephthalate and certain amounts (preferably <50 mol%) of naphthalene-2,6-dicarboxylic acid is used in the base layer (B).In this case, the producibility of the film is particularly good. Thebase layer (B) then comprises substantially a polyester copolymer whichis composed predominantly of terephthalic acid and isophthalic acidunits and/or terephthalic acid and naphthalene-2,6-dicarboxylic acidunits and of ethylene glycol units. The particularly preferredcopolyesters which provide the desired properties of the film are thosewhich are composed of terephthalate and isophthalate units and ofethylene glycol units.

The polyesters can be prepared by the transesterification process. Inthis process, the starting materials are dicarboxylic esters and diolswhich are reacted with the customary transesterification catalysts suchas salts of zinc, calcium, lithium and manganese. The intermediates arethen polycondensed in the presence of generally customarypolycondensation catalysts such as antimony trioxide, titanium oxides oresters, or else germanium compounds. The preparation may equally be bythe direct esterification process in the presence of polycondensationcatalysts. This process starts directly from the dicarboxylic acids andthe diols.

In a further embodiment, the base layer (B) and/or, if appropriate,another additional layer comprise at least one white pigment andoptionally an optical brightener.

To achieve the desired whiteness and the desired low transparency (lighttransmission), the base layer (B) comprises an inorganic white pigmentor a polymer which is substantially incompatible with polyester or acombination of the two. Useful inorganic white pigments are, forexample, titanium dioxide, calcium carbonate, barium sulfate, zincsulfide or zinc oxide. In order to attain the desired whiteness(preferably >60) and the desired low transparency (preferably <60%), thebase layer (B) should have a high filler content.

In a preferred embodiment, the sole coloring pigment used is titaniumdioxide (TiO₂). Preference is given to adding it to the original rawmaterial as an extrusion masterbatch (the titanium dioxide concentrationis distinctly higher here than in the biaxially oriented film). Typicalvalues for the TiO₂ concentration in the extrusion masterbatch are 50%by weight of titanium dioxide. The titanium dioxide may be either of therutile type or of the anatase type. Preference is given to usingtitanium dioxide of the rutile type. The particle size of the titaniumdioxide is generally between 0.05 and 0.5 μm, preferably between 0.1 and0.3 μm. The incorporated TiO₂ pigments in the base layer (B) impart tothe film a brilliant white appearance. The TiO₂ concentration to achievethe desired whiteness (>60) and the desired low transparency (<60%) isgenerally above 3% by weight but below 20% by weight, preferably above4% by weight but below 18% by weight and most preferably above 5% byweight but below 16% by weight, based on the total weight of the baselayer (B).

In a further preferred embodiment, the base layer (B) comprises at leastbarium sulfate as a pigment. The barium sulfate concentration to achievethe desired whiteness (>60) and the desired low transparency (<60%) isgenerally from 0.2 to 40% by weight, preferably from 0.5 to 30% byweight, more preferably from 1 to 25% by weight, based on the weight of(B) (see above). The average particle size of the barium sulfate isrelatively small and is preferably in the range from 0.1 to 5 μm, morepreferably in the range from 0.2 to 3 μm (Sedigraph method). The densityof the barium sulfate used is preferably between 4 and 5 g/cm³.Preference is given to metering the barium sulfate to the thermoplasticas an extrusion masterbatch in the film production. In a preferredembodiment, precipitated barium sulfate types are used. Precipitatedbarium sulfate is obtained from barium salts and sulfates or sulfuricacid as a finely divided colorless powder whose particle size can becontrolled by the precipitation conditions. Precipitated barium sulfatescan be prepared by the customary processes which are described inKunststoff Journal 8, No. 10, 30–36 and No. 11, 31–36 (1974). In aparticularly preferred embodiment, the main constituent present in thebase layer of the inventive film is a crystallizable polyethyleneterephthalate and from 1 to 25% by weight of precipitated bariumsulfate, appropriately having a particle diameter of from 0.4 to 1 μm,and particular preference is given to BLANC FIXE® XR-HX or BLANC FIXE®HXH from Sachtleben Chemie, Germany.

For a further increase in the whiteness, it is possible to add suitableoptical brighteners to the base layer (B), in which case the opticalbrightener is used in amounts in the range from 10 to 50 000 ppm, inparticular from 20 to 30 000 ppm, more preferably from 50 to 25 000 ppm,based on the weight (B). Preference is given to also metering theoptical brightener to the thermoplastic as an extrusion masterbatch inthe film production. The inventive optical brighteners are capable ofabsorbing UV rays in the range from 360 to 380 nm and emitting themagain as longer-wavelength, visible blue-violet light. Suitable opticalbrighteners are, for example, bisbenzoxazoles, phenylcoumarins andbisstearylbiphenyls, in particular phenylcoumarin; particular preferenceis given to triazinephenylcoumarin which is obtainable under the productname TINOPAL® from Ciba-Geigy, Basle, Switzerland, or HOSTALUX® KS(Clariant, Germany) and also EASTOBRITE® OB-1 (Eastman).

Where appropriate, it is also possible to add to the base layer (B), inaddition to the optical brightener, polyester-soluble blue dyes.Suitable blue dyes have been found to be, for example, cobalt blue,ultramarine blue and anthraquinone dyes, in particular SUDAN BLUE® 2(BASF, Ludwigshafen, Federal Republic of Germany). The blue dyes areused in amounts of from 10 to 10 000 ppm, in particular from 20 to 5000ppm, more preferably from 50 to 1000 ppm, based on the weight of thecrystallizable thermoplastic.

According to the invention, titanium dioxide or the barium sulfate, theoptical brightener and, where appropriate, the blue dye may already havebeen metered in by the manufacturer of the thermoplastic raw material ormay be metered in the course of film production via masterbatchtechnology into the extruder, for example, for the base layer (B).Particular preference is given to adding the titanium dioxide or thebarium sulfate, the optical brightener and, where appropriate, the bluedye via masterbatch technonology. The additives are preferably dispersedfully in a solid carrier material. Useful carrier materials include thethermoplastic itself, for example the polyethylene terephthalate or elseother polymers which are sufficiently compatible with the thermoplastic.It is advantageous when the particle size and the bulk density of theextrusion masterbatch(es) are similar to the particle size and the bulkdensity of the thermoplastic, so that a homogeneous distribution andtherefore a homogeneous whiteness and thus a homogeneous transparencyare achieved.

In a further favorable embodiment, the base layer (B), to achieve thedesired whiteness and the desired low transparency, comprises a polymerincompatible with polyester (anti-PET polymer). The preferredconcentrations of anti-PET polymer in (B) are from 4 to 50% by weight.

Examples of suitable incompatible polymers (anti-PET polymers) arepolymers based on ethylene (e.g. LLDPE, HDPE), propylene (PP),cycloolefins (CO), amides (PA) or styrene (PS). In a preferredembodiment, the polyester-incompatible polymer (anti-PET polymer) usedis a copolymer. Examples thereof are copolymers based on ethylene(C2/C3, C2/C3/C4 copolymers), propylene (C2/C3, C2/C3/C4 copolymers),butylene (C2/C3, C2/C3/C4 copolymers) or based on cycloolefins(norbornene/ethylene, tetracyclododecene/ethylene copolymers). In one ofthe particularly preferred embodiments, the polyester-incompatiblepolymer (anti-PET polymer) is a cycloolefin copolymer (COC). Suchcycloolefin copolymers are described, for example, in EP-A-1 068 949 orin JP 05-009319, which are incorporated herein by reference.

Among the cycloolefin copolymers, preference is given in particular tothose which comprise polymerized units of polycyclic olefins having anorbornene basic structure, more preferably norbornene ortetracyclododecene. Particular preference is given to cycloolefincopolymers (COC) which contain polymerized units of acyclic olefins, inparticular ethylene. Very particular preference is given tonorbornene/ethylene and tetracyclododecene/ethylene copolymers whichcontain from 5 to 80% by weight of ethylene units, preferably from 10 to60% by weight of ethylene units (based on the mass of the copolymer).

The cycloolefin polymers generally have glass transition temperaturesbetween −20 and 400° C. To achieve a white base layer (B) having lowtransparency, particularly suitable cycloolefin copolymers (COC) arethose which have a glass transition temperature T_(g) of greater than70° C., preferably greater than 90° C. and in particular greater than110° C. The viscosity number (decalin, 135° C., DIN 53 728) isappropriately between 0.1 and 200 ml/g, preferably between 50 and 150ml/g. The glass transition temperature is appropriately selected in sucha way that the COCs have a vacuole-inducing action.

The cycloolefin copolymers (COC) are prepared, for example, byheterogeneous or homogeneous catalysis with organometallic compounds andis described in a multitude of documents. Suitable catalyst systemsbased on mixed catalysts of titanium or vanadium compounds incombination with aluminum organyls are described in DD 109 224, DD 237070 and EP-A-0 156 464.

EP-A-0 283 164, EP-A-0 407 870, EP-A-0 485 893 and EP-A-0 503 422describe the preparation of cycloolefin copolymers (COC) with catalystsbased on soluble metallocene complexes. Particular preference is givento using cycloolefin copolymers prepared with catalysts which are basedon soluble metallocene complexes. Such COCs are commercially obtainable;for example TOPAS® (Ticona, Frankfurt).

To achieve the desired whiteness (>60) and the desired low transparency(<60%), the base layer (B) in the preferred embodiment comprises acycloolefin copolymer (COC) in a minimum amount of 2% by weight,preferably in an amount of from 4 to 50% by weight and more preferablyfrom 6 to 40% by weight, based on the weight of the base layer (B). Forthe preferred embodiment of the present invention, it is essential thatthe cycloolefin copolymer (COC) is not compatible with the polyester anddoes not form a homogeneous mixture with it. The COC is incorporatedinto the base layer either as a pure granule or as a granulatedconcentrate (masterbatch) by premixing the polyester granule or powderwith the COC or the COC masterbatch and subsequently feeding to theextruder. In the extruder, the components are mixed further and heatedto processing temperature. It is appropriate that the extrusiontemperature is above the glass transition temperature T_(g) of the COC,generally at least 5 K, preferably from 10 to 180 K, in particular from15 to 150 K, above the glass transition temperature T_(g) of thecycloolefin copolymer (COC).

In another preferred embodiment, the base layer (B) of the film, toimprove the whiteness and reduce the transparency, may contain acombination at least one white pigment, a substantiallypolyester-incompatible polymer (anti-PET polymer) and optionally anoptical brightener. Particular preference is given to optimizingwhiteness and transparency of the base layer (B) by a combination oftitanium dioxide or barium sulfate and a cycloolefin copolymer (COC),the cycloolefin copolymer (COC) having a glass transition temperatureT_(g) of greater than 70° C., preferably greater than 90° C. and inparticular greater than 110° C.

The film of the present invention has an at least two-layer structure.In that case, it includes the base layer (B) and the inventive sealableand peelable outer layer (A) applied to it by coextrusion.

The sealable and peelable outer layer (A) applied to the base layer (B)by coextrusion is composed predominantly, i.e. to an extent of at least80% by weight, of polyesters.

According to the invention, the heatsealable and peelable outer layer(A) comprises polyesters based on aromatic and aliphatic acids andpreferably aliphatic diols. In addition, the outer layer (A) comprisesinorganic and/or organic particles in a concentration of preferably from2 to 10% by weight.

In the preferred embodiment, polyesters are copolyesters or blends ofhomo- and copolyesters or blends of different copolyesters which areformed on the basis of aromatic and aliphatic dicarboxylic acids andaliphatic diols.

Examples of the aromatic dicarboxylic acids which can be used inaccordance with the invention are terephthalic acid, isophthalic acid,phthalic acid and naphthalene-2,6-dicarboxylic acid.

Examples of the aliphatic dicarboxylic acids which can be used inaccordance with the invention are succinic acid, glutaric acid, adipicacid, pimelic acid, suberic acid, azelaic acid and sebacic acid.

Examples of the aliphatic diols which can be used in accordance with theinvention are ethylene glycol, 1,3-propanediol, 1,3-butanediol,1,4-butanediol, 1,5-pentanediol, 2,2-dimethyl-1,3-propanediol,diethylene glycol, triethylene glycol and 1,4-cyclohexanedimethanol.

The polyester for the outer layer (A) is preferably prepared from twopolyesters I and II.

The proportion of the polyester I which includes one or more aromaticdicarboxylates and one or more aliphatic alkylenes in the outer layer(A) is from 0 to 50% by weight. In the preferred embodiment, theproportion of the polyester I is from 5 to 45% by weight, and in theparticularly preferred embodiment, it is from 10 to 40% by weight.

In general, the polyester I of the inventive outer layer (A) is based onthe following dicarboxylates and alkylenes, based in each case on thetotal amount of dicarboxylate or total amount of alkylene:

-   from 70 to 100 mol %, preferably from 72 to 95 mol % and more    preferably from 74 to 93 mol %, of terephthalate;-   from 0 to 30 mol %, preferably from 5 to 28 mol % and more    preferably from 7 to 26 mol %, of isophthalate;-   more than 50 mol %, preferably more than 65 mol % and more    preferably more than 80 mol %, of ethylene units.

Any remaining fractions present stem from other aromatic dicarboxylicacids and other aliphatic diols, as have already been listed above forthe base layer (B).

Very particular preference is given to those copolyesters in which theproportion of terephthalate units is from 74 to 88 mol %, thecorresponding proportion of isophthalate units is from 12 to 26 mol %(the dicarboxylate proportions adding up to 100 mol %) and theproportion of ethylene units is 100 mol %. In other words, they arepolyethylene terephthalate/isophthalate.

In a further preferred embodiment, the polyester I includes a mixturewhich comprises a copolyester composed of terephthalate, isophthalateand ethylene units, and an aromatic polyester homopolymer, e.g. apolybutylene terephthalate.

According to the present invention, the proportion of polyester II inthe outer layer (A) is from 50 to 100% by weight. In the preferredembodiment the proportion of polyester II is from 55 to 95% by weightand in the particularly preferred embodiment it is from 60 to 90% byweight.

The polyester II preferably includes a copolymer of aliphatic andaromatic acid components, in which the aliphatic acid components arepreferably from 20 to 90 mol %, in particular from 30 to 70 mol % andmore preferably from 35 to 60 mol %, based on the total acid amount ofthe polyester II. The remaining dicarboxylate content up to 100 mol %stems from aromatic acids, preferably terephthalic acid and/orisophthalic acid, and also, among the glycols, from aliphatic orcycloaliphatic or aromatic diols, as have already been described indetail above with regard to the base layer.

In general, the polyester II of the inventive outer layer (A) is basedpreferably at least on the following dicarboxylates and alkylenes, basedin each case on the total amount of dicarboxylate or the total amount ofalkylene:

from 20 to 90 mol %, preferably from 30 to 65 mol % and more preferablyfrom 35 to 60 mol %, of azelate;

from 0 to 50 mol %, preferably from 0 to 45 mol % and more preferablyfrom 0 to 40 mol %, of sebacate;

from 0 to 50 mol %, preferably from 0 to 45 mol % and more preferablyfrom 0 to 40 mol %, of adipate;

from 10 to 80 mol %, preferably from 20 to 70 mol % and more preferablyfrom 30 to 60 mol %, of terephthalate;

from 0 to 30 mol %, preferably from 3 to 25 mol % and more preferablyfrom 5 to 20 mol %, of isophthalate;

more than 30 mol %, preferably more than 40 mol % and more preferablymore than 50 mol %, of ethylene or butylene.

Any remaining fractions present stem from other aromatic dicarboxylicacids and other aliphatic diols, as have already been listed above forthe base layer (B), or else from hydroxycarboxylic acids such ashydroxybenzoic acid or the like.

The presence of preferably at least 10 mol % of aromatic dicarboxylicacid ensures that the polymer II can be processed without adhesion, forexample in the coextruder or in the longitudinal stretching.

The outer layer (A) preferably comprises a mixture of the polyesters Iand II. Compared to the use of only one polyester with comparablecomponents and comparable proportions of the components, a mixture hasthe following advantages:

The mixture of the two polyesters I and II, from the aspect of theparticular glass transition temperatures (T_(g)s) is easier to process(to extrude). As investigations have shown, the mixture of a polymerhaving a high T_(g) (polyester I) and a polymer having a low T_(g)(polyester II) has a lesser tendency to adhere in the intake of thecoextruder than a single polymer having a correspondingly mixed T_(g).

The polymer production is simpler, because the number of meteringstations available for the starting materials is generally notunlimited.

Moreover, from a practical aspect, the desired peeling properties can beadjusted more individually with the mixture than when a single polyesteris used.

The addition of particles (see below) is also simpler in the case ofpolyester I than in the case of polyester II.

Appropriately, the glass transition temperature of polyester I is morethan 50° C. The glass transition temperature of polyester I ispreferably more than 55° C. and more preferably more than 60° C. Whenthe glass transition temperature of polyester I is less than 50° C., thefilm in some circumstances cannot be produced in a reliable process. Thetendency of the outer layer (A) to adhere, for example to rolls, may beso high that frequent film breaks, in particular in the longitudinalstretching, have to be expected. When this happens, the film can windaround the rolls in the longitudinal stretching, which can lead toconsiderable damage to the machine. In the extrusion, such a polyesteradheres readily to the metallic walls and thus leads to blockages.

Appropriately, the glass transition temperature of polyester II is lessthan 20° C. The glass transition temperature is preferably less than 15°C. and more preferably less than 10° C. When the glass transitiontemperature of polyester II is greater than 20° C., the film has anincreased tendency to start to tear or tear off when pulled off thetray, which is undesired.

In a preferred embodiment, the heatsealable and peelable outer layer (A)additionally comprises a polymer which is incompatible with polyester(anti-PET polymer). According to the present invention, the proportionof the polyester-incompatible polymer (anti-PET polymer) is from 2 to18% by weight, based on the mass of the outer layer (A). In a preferredembodiment, the proportion of the polymer is from 5 to 17% by weight andin the particularly preferred embodiment it is from 7 to 16% by weight,likewise based on the mass of the outer layer (A).

Examples of suitable incompatible polymers have already been describedabove for the base layer (B). In one of the particularly preferredembodiments, the polymer incompatible with polyester is a cycloolefincopolymer (COC). As already described for the base layer (B), veryparticular preference is given here also to norbornene/ethylene andtetracyclododecene/ethylene copolymers which contain from 5 to 80% byweight of ethylene units, preferably from 10 to 60% by weight ofethylene units (based on the mass of the copolymer).

The cycloolefin polymers generally have glass transition temperaturesbetween −20 and 400° C. To improve the peeling properties of the outerlayer (A), particularly suitable cycloolefin copolymers (COCs) are thosewhich have a glass transition temperature of less than 160° C.,preferably less than 120° C. and more preferably less than 80° C. Theglass transition temperature should preferably be above 50° C., withpreference above 55° C., in particular above 60° C. The viscosity number(decalin, 135° C., DIN 53 728) is appropriately between 0.1 and 200ml/g, preferably between 50 and 1.50 ml/g.

For the peelable outer layer (A) too, particular preference is given tousing cycloolefin copolymers prepared with catalysts which are based onsoluble metallocene complexes. Such COCs are commercially obtainable;for example TOPAS® (Ticona, Frankfurt).

According to the invention, the heatsealable and peelable outer layer(A) comprises inorganic and/or organic particles. According to thepresent invention, the proportion of particles is preferably from 2 to10% by weight, based on the mass of the outer layer (A). In a preferredembodiment, the proportion of particles is from 2.5 to 9% by weight andin the particularly preferred embodiment it is from 3.0 to 8% by weight,likewise based on the mass of the outer layer (A).

In contrast, when the outer layer (A) of the film contains particles ina concentration of less than 2% by weight, there is generally nopositive influence on the removal performance of the film from the traynor a positive influence on self-venting; the film tends to start totear or to tear off. In contrast, when the outer layer (A) of the filmcontains particles in a concentration of more than 10% by weight, thesealing of the film is too greatly weakened.

It has been found to be advantageous when the particles are present in acertain size, in a certain concentration and in a certain distribution.In addition, it is also possible to add mixtures of two and moredifferent particle systems or mixtures of particle systems in the samechemical composition, but different particle size, to the outer layer(A).

Customary particles (also referred to as pigments or antiblockingagents) are inorganic and/or organic particles, for example calciumcarbonate, amorphous silica, talc, magnesium carbonate, bariumcarbonate, calcium sulfate, barium sulfate, lithium phosphate, calciumphosphate, magnesium phosphate, alumina, lithium fluoride, calcium,barium, zinc or manganese salts of the dicarboxylic acids used, carbonblack, titanium dioxide, kaolin or crosslinked polystyrene or acrylateparticles. The particles may be added to the layer in the particularadvantageous concentrations, for example as a glycolic dispersion duringthe polycondensation or via masterbatches in the course of theextrusion.

Particles which are preferred in accordance with the invention aresynthetic, amorphous SiO₂ particles in colloidal form. These particlesare bound into the polymer matrix in an outstanding manner. To(synthetically) produce the SiO₂ particles (also known as silica gel),sulfuric acid and sodium silicate are initially mixed with one anotherunder controlled conditions to form hydrosol. This eventually forms ahard, transparent mass which is known as a hydrogel. After separation ofthe sodium sulfate formed as a by-product by a washing process, thehydrogel can be dried and further processed. Control of the washingwater pH and the drying conditions can be used to vary the importantphysical parameters, for example pore volume, pore size and the size ofthe surface of the resulting silica gel. The desired particle size (forexample the d₅₀ value) and the desired particle size distribution (forexample the SPAN98) are obtained by suitable grinding of the silica gel(for example mechanically or hydromechanically). Such particles can beobtained, for example, via Grace, Fuji, Degussa or Ineos.

The particles preferably have an average particle diameter d₅₀ of from3.0 to 12.0 μm, in particular from 3.5 to 11.0 μm and more preferablyfrom 4.0 to 10.0 μm. When particles having a diameter which is below 3.0μm are used, there is generally no positive influence of the particleson self-venting.

In the heatsealable and peelable outer layer (A), the ratio of particlesize d₅₀ to layer thickness d_(A) of the outer layer (A) is preferably≧1.5. In particular, the diameter/layer thickness ratio is at least 1.7and more preferably at least 2.0. In these cases, there is aparticularly positive influence of the particles on self-venting.

It has been found to be particularly advantageous to use particles inthe heatsealable and peelable outer layer (A) whose particle diameterdistribution has a degree of scatter which is described by a SPAN98 of≦2.0 (definition of SPAN98, see test method). Preference is given to aSPAN98 of ≦1.9 and particular preference to a SPAN98 of ≦1.8. Incontrast, when the outer layer (A) of the film comprises particles whoseSPAN98 is greater than 2.0, the optical properties and the sealingproperties of the film deteriorate.

Moreover, it has been found to be advantageous for the suitability as afilm with self-venting to adjust the roughness of the heatsealable andpeelable outer layer (A) in such a way that its R_(a) value ispreferably greater than 120 nm. The roughness R_(a) is preferablygreater than 160 nm and it is more preferably greater than 200 nm; theupper limit of the roughness should not exceed 400 nm, preferably 350nm, in particular 300 nm. This can be controlled via the selection ofthe particle diameters, their concentration and the variation of thelayer thickness.

In a particularly advantageous three-layer embodiment (ABC), the filmincludes the base layer (B), the inventive outer layer (A) and an outerlayer (C) on the opposite side to the outer layer (A). This outer layer(C) may include the polymers described for the base layer. To improvethe winding capability, the outer layer (C) comprises the customaryparticles (also known as “antiblocking agents”), as already describedfor outer layer (A). The particles of the outer layer (C) should have anaverage particle diameter d₅₀ (=median) of preferably from 1.5 to 6 μm.It has been found to be particularly appropriate to use particles havingan average particle diameter d₅₀ of from 2.0 to 5 μm and more preferablyfrom 2.5 to 4 μm. The particles of the outer layer (C) should preferablyhave a degree of scatter which is described by a SPAN98 of ≦2.0.Preference is given to the SPAN98 being ≦1.9 and particular preferenceto the SPAN98 being ≦1.8. The particles of the outer layer (C) shouldgenerally be present in a concentration of from 0.1 to 0.5% by weight.The concentration of the particles is preferably from 0.12 to 0.4% byweight and more preferably from 0.15 to 0.3% by weight.

The outer layer (C) increases the gloss of the film and, in the case ofa white film, prevents the attrition of the highly filled white baselayer (B). The gloss of the film surface (C) in a three-layer film ispreferably greater than 100 (measured to DIN 67530 with reference toASTM-D 523-78 and ISO 2813 with angle of incidence 20°). In a preferredembodiment, the gloss of this side is more than 110 and in aparticularly preferred embodiment more than 120. This film surface istherefore especially suitable for a further functional coating, forprinting or for metallization.

Between the base layer and the outer layers may optionally be disposedanother intermediate layer. This may in turn include the polymersdescribed for the base layer. In a particularly preferred embodiment,the intermediate layer include the polyesters used for the base layer.The intermediate layer may also comprise the customary additivesdescribed below. The thickness of the intermediate layer is generallygreater than 0.3 μm and is preferably in the range from 0.5 to 15 μm, inparticular in the range from 1.0 to 10 μm, more preferably in the rangefrom 1.0 to 5 μm.

In the case of the two-layer and the particularly advantageousthree-layer embodiment of the inventive film, the thickness of the outerlayer (A) is preferably in the range from 0.7 to 8.0 μm, in particularin the range from 1.0 to 7.0 μm and more preferably in the range from1.3 to 6.0 μm. When the thickness of the outer layer (A) is more than8.0 μm, the peeling force rises markedly and is no longer within thepreferred range. Furthermore, the peeling performance of the film isimpaired. In contrast, when the thickness of the outer layer (A) is lessthan 0.7 μm, the film generally no longer has the desired peelingproperties.

The thickness of the other, nonsealable outer layer (C) may be the sameas the outer layer (A) or different; its thickness is generally between0.5 and 5 μm.

The total thickness of the inventive polyester film may vary within widelimits. It is preferably from 3 to 200 μm, in particular from 4 to 150μm, preferably from 5 to 100 μm, and the layer (B) has a proportion ofpreferably from 45 to 97% of the total thickness.

The base layer and the other layers may additionally comprise customaryadditives, for example stabilizers (UV, hydrolysis), flame-retardantsubstances or fillers. They are appropriately added to the polymer or tothe polymer mixture before the melting.

The present invention also provides a process for producing the film. Toproduce the inventive peelable outer layer (A), the particular polymers(polyester I, polyester II, optionally further polymers, for examplepolyester-incompatible polymer (anti-PET polymer), masterbatch(es) forparticles) are appropriately fed directly to the extruder for the outerlayer (A). The materials can be extruded at from about 200 to 280° C.From a process engineering point of view (mixing of the differentcomponents), it has been found to be particularly favorable when theextrusion of the polymers for the outer layer (A) is carried out using atwin-screw extruder having degassing means.

The polymers for the base layer (B) and for the further outer layer (C)which is possibly present and, where appropriate, the intermediate layerare appropriately fed to the (coextrusion) system via further extruders.The melts are shaped to flat melt films in a multilayer die and layeredon top of one another. Subsequently, the multilayer film is drawn offwith the aid of a chill roll and optionally further rolls andsolidified.

The biaxial stretching of the film is generally carried outsequentially. Simultaneous stretching of the film is also possible, butis not necessary. In the sequential stretching, preference is given tostretching first in longitudinal direction (i.e. in machinedirection=MD) and then in transverse direction (i.e. at right angles tomachine direction=TD). The stretching in longitudinal direction can becarried out with the aid of two rolls rotating at different rates inaccordance with the desired stretching ratio. For transverse stretching,an appropriate tenter frame is generally used.

The temperature at which the stretching is carried out can be variedwithin a relatively wide range and depends on the desired properties ofthe film. In general, the stretching is carried out in the longitudinaldirection (machine direction orientation=MDO) in a temperature range offrom approx. 60 to 130° C. (heating temperatures from 60 to 130° C.),and in transverse direction (transverse direction orientation=TDO) in atemperature range from approx. 90° C. (commencement of stretching) to140° C. (end of stretching). The longitudinal stretching ratio ispreferably in the range from 2:1 to 5.5:1, in particular from 2.3:1 to5.0:1. The transverse stretching ratio is preferably in the range from2.4:1 to 5.0:1, in particular from 2.6:1 to 4.5:1.

The preferred temperature range at which the biaxial stretching iscarried out is from 60 to 120° C. in the longitudinal stretching (MDO).The heating temperatures of the film in the longitudinal stretching arein the range from 60 to 115° C. In the transverse stretching (TDO), thetemperatures of the film are preferably in the range from 90° C.(commencement of stretching) to 140° C. (end of stretching). Thelongitudinal stretching ratio in this preferred temperature range is inthe range from 2.0:1 to 5.0:1, preferably from 2.3:1 to 4.8:1. Thetransverse stretching ratio is generally in the range from 2.4:1 to5.0:1, preferably from 2.6:1 to 4.5:1.

The particularly preferred temperature range in which the biaxialstretching is carried out in the case of the longitudinal stretching(MDO) is from 60 to 110° C. The heating temperatures of the film in thelongitudinal stretching are in the range from 60 to 105° C. In thetransverse stretching (TDO), the temperatures of the film are in therange from 90° C. (beginning of the stretching) to 140° C. (end of thestretching). The longitudinal stretching ratio in this preferredtemperature range is in the range from 2.0:1 to 4.8:1, preferably from2.3:1 to 4.6:1. The transverse stretching ratio is generally in therange from 2.4:1 to 5.0:1, preferably from 2.6:1 to 4.5:1.

The preferred and especially the particularly preferred temperatures inthe MDO particularly effectively take account of the adherent behaviorof outer layer (A) to rolls (metallic, ceramic or particularly coatedroll surfaces).

Before the transverse stretching, one or both surfaces of the film canbe coated inline by the processes known per se. The inline coating maylead, for example, to improved adhesion between a metal layer or aprinting ink and the film, to an improvement in the antistaticperformance, in the processing performance or else to furtherimprovement of barrier properties of the film. The latter is obtained,for example, by applying barrier coatings such as EVOH, PVOH or thelike. In that case, preference is given to applying such layers to thenonsealable surface, for example the surface (C) of the film.

In the subsequent heat-setting, the film is kept at a temperature offrom approx. 150 to 200° C. over a period of from about 0.1 to 10 s toachieve the inventive shrinkage. The fixing time set and the fixingtemperature set are governed by the desired shrinkage alone. When acomparatively high shrinkage is required, the fixing time should be setcomparatively short and the fixing temperature set comparatively low.Subsequently, the film is wound up in a customary manner.

A further advantage of the invention is that the production costs of theinventive film are not substantially above those of a film made ofstandard polyester. In addition, it is guaranteed that, in the course ofthe production of the film, offcut material which arises intrinsicallyin the operation of the film production can be reused for the filmproduction as regrind in an amount of up to approx. 60% by weight,preferably from 5 to 50% by weight, based in each case on the totalweight of the film, without the physical properties of the film beingsignificantly adversely affected.

The film according to the invention is, for example, outstandinglysuitable for packaging foods and other consumable goods, in particularin the packaging of foods and other consumable goods in trays in whichpeelable polyester films are used for opening the packaging.

The table which follows (table 1) once again summarizes the mostimportant preferred film properties.

TABLE 1 Inventive More Test range Preferred preferred Unit method Outerlayer (A) or film Proportion of units in the inventive polyester,  12 to89  30 to 84  40 to 82 mol % formed from aromatic dicarboxylic acidsProportion of units in the inventive polyester,  11 to 88  16 to 70  18to 60 mol % formed from aliphatic dicarboxylic acids Polyester I   0 to50   5 to 45  10 to 40 % by wt. Polyester II  50 to 100  55 to 95  60 to90 % by wt. Particle diameter d₅₀ 3.0 to 12 3.5 to 11 4.0 to 10.0 μmFiller concentration 2.0 to 10.0 2.5 to 9.0 3.0 to 8.0 % by wt.Thickness of the outer layer A 0.7 to 8.0 1.0 to 7.0 1.3 to 6.0 μmParticle diameter/layer thickness ratio >/=1.5 >/=1.7 >/=2.0 Shrinkageof the film, at least in one direction >/=5 >/=8 >/=10 % DIN 40634Properties Thickness of the film   3 to 200   4 to 150   5 to 100 μmMinimum sealing temperature of OL (A) with respect 165 160 155 ° C. toPET trays Seal seam strength of OL (A) with respect to PET 1.5 to 8 2.0to 8 2.5 to 8 N/15 mm trays Gloss of the outer layers A and C >70and >100 >75 and >110 >80 and >120 DIN 67530 Opacity of the film,transparent version <20 <16 <12 % ASTM-D 1003-52 OL: Outer layer, >/=:greater than/equal to

To characterize the raw materials and the films, the followingmeasurement methods were used for the purposes of the present invention:

Measurement of the Average Diameter d₅₀

The determination of the average diameter d₅₀ was carried out by meansof laser on a Malvern Master Sizer (from Malvern Instruments Ltd., UK)by means of laser scanning (other measuring instruments are, forexample, Horiba LA 500 or Sympathec Helos, which use the same measuringprinciple). To this end, the samples were introduced together with waterinto a cuvette and this was then placed in the measuring instrument. Thedispersion is scanned by means of a laser and the signal is used todetermine the particle size distribution by comparison with acalibration curve. The particle size distribution is characterized bytwo parameters, the median value d₅₀ (=measure of the position of theaverage value) and the degree of scatter, known as the SPAN98 (=measureof the scatter of the particle diameter). The measuring procedure isautomatic and also includes the mathematical determination of the d₅₀value. The d₅₀ value is determined by definition from the (relative)cumulative curve of the particle size distribution: the point at whichthe 50% ordinate value cuts the cumulative curve provides the desiredd₅₀ value (also known as median) on the abscissa axis.

Measurement of SPAN98

The determination of the degree of scatter, the SPAN98, was carried outwith the same measuring instrument as described above for thedetermination of the average diameter d₅₀. The SPAN98 is defined asfollows:

${SPAN98} = \frac{d_{98} - d_{10}}{d_{50}}$

The basis of the determination of d₉₈ and d₁₀ is again the (relative)cumulative curve of the particle size distribution (see above“Measurement of the average diameter d₅₀”). The point at which the 98%ordinate value cuts the cumulative curve provides the desired d₉₈ valuedirectly on the abscissa axis and the point at which the 10% ordinatevalue cuts the cumulative curve provides the desired d₁₀ value on theabscissa axis.

SV Value

The SV value of the polymer was determined by the measurement of therelative viscosity (ηrel) of a 1% solution in dichloroacetic acid in anUbbelohde viscometer at 25° C. The SV value is defined as follows:SV=(ηrel−1)·1000.Glass Transition Temperatures T_(g)

The glass transition temperature T_(g) was determined using film sampleswith the aid of DSC (differential scanning calorimetry). The instrumentused was a Perkin-Elmer DSC 1090. The heating rate was 20 K/min and thesample weight approx. 12 mg. In order to eliminate the thermal history,the samples were initially preheated to 300° C., kept at thistemperature for 5 minutes and then subsequently quenched with liquidnitrogen. The thermogram was used to find the temperature for the glasstransition T_(g) as the temperature at half of the step height.

Seal Seam Strength (Peeling Force)

To determine the seal seam strength, a film strip (100 mm long×15 mmwide) is placed on an appropriate strip of the tray and sealed at theset temperature of >140° C., a sealing time of 0.5 s and a sealingpressure of 4 bar (HSG/ET sealing unit from Brugger, Germany, sealingjaw heated on both sides). In accordance with FIG. 2, the sealed stripsare clamped into the tensile testing machine (for example from Zwick,Germany) and the 180° seal seam strength, i.e. the force required toseparate the test strips, was determined at a removal rate of 200mm/min. The seal seam strength is quoted in N per 15 mm of film strip(e.g. 3 N/15 mm).

Determination of the Minimum Sealing Temperature

The Brugger HSG/ET sealing unit as described above for the measurementof the seal seam strength is used to produce heatsealed samples (sealseam 15 mm×100 mm), and the film is sealed at different temperatureswith the aid of two heated sealing jaws at a sealing pressure of 4 barand a sealing time of 0.5 s. The 180° seal seam strength was measured asfor the determination of the seal seam strength. The minimum sealingtemperature is the temperature at which a seal seam strength of at least1.0 N/15 mm is attained.

Roughness

The roughness R_(a) of the film was determined to DIN 4768 at a cutoffof 0.25 mm. It was not measured on a glass plate, but rather in a ring.In the ring method, the film is clamped into a ring, so that neither ofthe two surfaces touches a third surface (for example glass).

Whiteness

The whiteness is determined according to Berger, by generally placingmore than 20 film plies one on top of another. The whiteness isdetermined with the aid of the ®ELREPHO electrical remission photometerfrom Zeiss, Oberkochem (Germany), standard illuminant C, 2° normalobserver. The whiteness is defined as W=RY+3RZ−3RX. W=whiteness, RY, RZ,RX=appropriate reflection factors when a Y, Z and X color measurementfilter is used. The white standard used is a barium sulfate presscake(DIN 5033, part 9). A comprehensive description is given, for example,in Hansl Loos, “Farbmessung” [Color measurement], Verlag Beruf undSchule, Itzehoe (1989).

Opacity

The opacity light transmission/transparency refers to the ratio of thetotal amount of light transmitted to the amount of incident light. Theopacity is measured to ASTM D 1003 with the “Hazegard plus” instrument(from Pausch-Messtechnik, Haan, Germany).

Gloss

The gloss of the film was determined to DIN 67530. The reflector valuewas measured as a characteristic optical parameter for the surface of afilm. Based on the standards ASTM-D 523-78 and ISO 2813, the angle ofincidence was set to 20°. A light beam hits the flat test surface at theangle of incidence set and is reflected or scattered by it. The lightbeams incident on the photoelectronic detector are displayed as aproportional electrical quantity. The measurement is dimensionless andhas to be quoted together with the angle of incidence.

Tensile Strain at Break

The tensile strain at break of the film was measured to DIN 53455. Thetesting rate is 1%/min; 23° C.; 50% relative humidity.

Modulus of Elasticity

The modulus of elasticity of the film was measured to DIN 53457. Thetesting rate is 1%/min; 23° C.; 50% relative humidity.

Shrinkage

The shrinkage of the film was determined to DIN 40634. The testingconditions are 100° C., 15 min.

The invention is illustrated hereinbelow with reference to examples.

EXAMPLE 1

Chips of polyethylene terephthalate were dried at 160° C. to a residualmoisture content of less than 50 ppm and fed to the extruder for thebase layer (B). Chips of polyethylene terephthalate and particles werelikewise fed to the extruder (twin-screw extruder with degassing) forthe nonsealable outer layer (C). In accordance with the processconditions listed in the table below, the raw materials were melted andhomogenized in the two respective extruders.

In addition, a mixture including polyester I, polyester II and SiO₂particles was prepared for the heatsealable and peelable outer layer(A). Table 2 specifies the particular proportions of the dicarboxylicacids and glycols present in the two polyesters I and II in mol % andthe particular proportions of the components present in the mixture in %by weight. The mixture was fed to the twin-screw extruder with degassingfor the sealable and peelable outer layer (A). In accordance with theprocess conditions detailed in the table below, the raw materials weremelted and homogenized in the twin-screw extruder.

By coextrusion in a three-layer die, the three melt streams were thenlayered one on top of the other and ejected via the die lip. Theresulting melt film was cooled and a three-layer film having ABCstructure was subsequently produced in a total thickness of 25 μm by astepwise orientation in longitudinal and transverse direction. Thethickness of the outer layer (A) is 3.0 μm. The thickness of the outerlayer (C) is 1.1 μm (cf. also table 2).

Outer layer (A), mixture of:

-   45% by weight of polyester I (=copolymer of 78 mol % of ethylene    terephthalate, 22 mol % of ethylene isophthalate) having an SV value    of 850. The glass transition temperature of polyester I is approx.    75° C. Polyester I additionally contains 10.0% by weight of ®Sylysia    440 (synthetic SiO₂, Fuji, Japan) having a particle diameter of    d₅₀=5.5 μm and a SPAN98 of 1.8. The ratio of particle diameter d₅₀    to outer layer thickness d_((A)) is 1.83:1 (cf. table 2);-   55% by weight of polyester II (=copolymer containing 40 mol % of    ethylene azelate, 50 mol % of ethylene terephthalate, 10 mol % of    ethylene isophthalate) having an SV value of 1000. The glass    transition temperature of polyester II is approx. 0° C.

Base layer (B):

-   100% by weight of polyethylene terephthalate having an SV value of    800

Outer layer (C), mixture of:

-   85% by weight of polyethylene terephthalate having an SV value of    800-   15% by weight of a masterbatch of 99% by weight of polyethylene    terephthalate (SV value of 800) and 1.0% by weight of SYLOBLOC® 44 H    (synthetic SiO₂, Grace, Worms), d₅₀=2.5 μm, SPAN98=1.9

The production conditions in the individual process steps were:

Extrusion Temperatures Layer A: 230 ° C. Layer B: 280 ° C. Layer C: 280° C. Temperature of the 20 ° C. takeoff roll Longitudinal Heatingtemperature 70–100 ° C. stretching Stretching temperature 102 ° C.Longitudinal stretching 3.8 ratio Transverse Heating temperature 100 °C. stretching Stretching temperature 130 ° C. Transverse stretching 3.5ratio Setting Temperature 160 ° C. Time 2 s

Table 3 shows the properties of the film. According to measurements(column 2), the minimum sealing temperature of the film with respect toCPET trays is 152° C. The film was sealed to the CPET trays at 160, 180and 200° C. (sealing pressure 4 bar, sealing time 0.5 s). Subsequently,strips of the bond of inventive film and CPET tray were pulled apart bymeans of a tensile strain tester in accordance with the aforementionedtest method (cf. FIG. 2). For all sealing temperatures, the filmsexhibited the desired peeling off from the tray according to FIG. 3 b.The seal seam strengths measured are listed in column 3. For all sealingtemperatures, peelable films were obtained. The seal seam strengths arewithin the lower range, i.e. the films can be removed from the traywithout force being applied. In addition, the film featured the desiredself-venting, had the required good optical properties, exhibited thedesired handling and the desired processing performance.

EXAMPLE 2

In comparison to example 1, the composition of the mixture for thesealable outer layer (A) was changed. The composition of the individualcomponents remained unchanged in comparison to example 1. The mixturenow includes the following raw material proportions:

-   polyester I=40% by weight-   polyester II=60% by weight

As a consequence of the higher proportion of polyester II in themixture, the process parameters in the longitudinal stretching weremodified. The new conditions for longitudinal stretching are listed inthe table below.

Longitudinal stretching Heating temperature 70–95 ° C. Stretchingtemperature 97 ° C. Longitudinal stretching ratio 3.7

The minimum sealing temperature of the film with respect to CPET traysis now 150° C. For all sealing temperatures, the films exhibited thedesired peeling off from the tray according to FIG. 3 b. The seal seamstrengths measured are listed in column 3. For all sealing temperatures,peelable films were again obtained. The seal seam strengths of theinventive films are higher than those in example 1. They are within amedium range, so that the film can be removed from the tray withoutsignificant force being applied. The self-venting, the opticalproperties, handling and the processing performance of the film were asin example 1.

EXAMPLE 3

In comparison to example 2, the composition of the mixture for thesealable outer layer (A) was changed. The composition of the individualcomponents remained unchanged in comparison to example 1. The mixturenow includes the following raw material proportions:

-   polyester I=30% by weight-   polyester II=70% by weight

As a consequence of the higher proportion of polyester II in themixture, the process parameters in the longitudinal stretching weremodified. The new conditions for longitudinal stretching are listed inthe table below.

Longitudinal stretching Heating temperature 70–90 ° C. Stretchingtemperature 93 ° C. Longitudinal stretching ratio 3.5

The minimum sealing temperature of the film with respect to CPET traysis now 149° C. For all sealing temperatures, the films exhibited thedesired peeling off from the tray according to FIG. 3 b. The seal seamstrengths measured are listed in column 3. For all sealing temperatures,peelable films were again obtained. The seal seam strengths of theinventive films are comparable to those from example 1. Theself-venting, the optical properties, handling and the processingperformance of the film were as in example 1.

EXAMPLE 4

In comparison to example 3, the composition of polyester II for thesealable outer layer (A) was changed. The mixture used in outer layer(A) now includes the following raw material proportions:

-   20% by weight of polyester I, identical to example 1 apart from the    concentration of the antiblocking agent. Polyester I now contains    20.0% by weight of SYLYSIA® synthetic SiO₂, Fuji, Japan) having a    particle diameter of d₅₀=5.5 μm and a SPAN98 of 1.8;-   80% by weight of polyester II, VITEL® 912, (polyester,    Bostik-Findley, USA; contains the dicarboxylic acid constituents    azelaic acid, sebacic acid, terephthalic acid, isophthalic acid and    further dicarboxylic acids approximately in the molar ratio    40/1/45/10/4, and, as the diol component, at least 60 mol % of    ethylene glycol). The glass transition temperature of polyester II    is approx. −1° C.

In addition, the base layer (B) was also changed. The base layer (B) nowincludes the following raw material proportions:

Base layer (B):

-   100% by weight of copolymer containing 12 mol % of ethylene    isophthalate and 88 mol % of ethylene terephthalate having an SV    value of 760.

The process parameters in the longitudinal stretching corresponded tothose in example 3. The minimum sealing temperature of the film producedin accordance with the invention to CPET trays is now 138° C. For allsealing temperatures, the films exhibited the desired peeling off fromthe tray according to FIG. 3 b. The seal seam strengths measured arelisted in column 3. For all sealing temperatures, peelable films wereagain obtained.

The seal seam strengths are within a medium range, so that the film canbe removed from the tray without great force being applied. Theself-venting, the optical properties, the handling and the processingperformance of the film were as in example 1.

COMPARATIVE EXAMPLE 1

In comparison to example 1, the composition of the sealable layer (A)was changed. In the outer layer (A), only the polyester I based onaromatic acids was used:

Outer layer (A):

-   100% by weight of polyester I (=copolymer of 78 mol % of ethylene    terephthalate and 22 mol % of ethylene isophthalate) having an SV    value of 850. The glass transition temperature of polyester I is    approx. 75° C. In addition, polyester I contains 5.0% of SYLYSIA®    430.

The production conditions in the individual process steps were adaptedin the longitudinal stretching to the glass transition temperature ofthe outer layer raw material:

Longitudinal stretching Heating temperature 70–115 ° C. Stretchingtemperature 120 ° C. Longitudinal stretching ratio 4

Table 3 shows the properties of the film. Even though the sealing layeris highly pigmented and the pigments constitute weak points in thesealing layer, a peelable film was not obtained for any of the sealingtemperatures specified. On removal of the film from the tray, the filmstarted to tear immediately and exhibited a force-distance diagramaccording to FIG. 3 a. The film exhibits weldable behavior and is thusunsuitable for the achievement of the object specified.

COMPARATIVE EXAMPLE 2

Example 1 from WO 02/26493 was reproduced. Table 3 shows the propertiesof the film. Although the film does exhibit the desired self-venting,the production is more than twice as expensive as the solutionsspecified above. In addition, there was distinct thread formation whenthe laminate was removed from the tray (possibly owing to the lack ofpigmentation and the excessively high content of aliphatic component).

COMPARATIVE EXAMPLE 3

Example 5 from EP-A-0 035 835 was reproduced. Table 3 shows theproperties of the film. A peelable film was not obtained for any of thespecified sealing temperatures. On removal of the film from the tray,the film started to tear immediately and exhibited a force-distancediagram according to FIG. 3 a. The film exhibits weldable behavior andis thus unsuitable for the achievement of the object specified.

COMPARATIVE EXAMPLE 4

Example 1 from EP-A-0 379 190 was reproduced. Table 3 shows theproperties of the film. A peelable film was not obtained for any of thespecified sealing temperatures. On removal of the film from the tray,the film started to tear immediately and exhibited a force-path diagramaccording to FIG. 3 a. The film exhibits weldable behavior and is thusunsuitable for the achievement of the object specified.

COMPARATIVE EXAMPLE 5

Example 22 from EP-A-0 379 190 was reproduced. Table 3 shows theproperties of the film. A peelable film was not obtained for any of thespecified sealing temperatures.

On removal of the film from the tray, the film started to tearimmediately and exhibited a force-path diagram according to FIG. 3 a.The film exhibits weldable behavior and is thus unsuitable for theachievement of the object specified.

The composition of the films is specified in table 2, the filmproperties measured in table 3.

TABLE 2 PI/PII/ anti-PET PI/PII/ polymer Composition of anti-PET glasspolyester I Composition of polyester II polymer transition TA IA EG NGAzA SeA AdA TA IA EG BD FA ratios temperatures mol % mol % % by wt. ° C.Examples 1  78 22 100 40 50 10 100 45/55 75/0 2  78 22 100 40 50 10 10040/60 75/0 3  78 22 100 40 50 10 100 30/70 75/0 4  78 22 100 40  1 4510 >60 4 20/80 75/−1 Comparative 1  78 22 100 — — — — — — — — 100/0/0/75 Examples 2 — — — 45 — — 55 — 100 — — — 75 3  82 18 100 — — — — — — —— 100/0/0/ 75 4 — — — 10 90 100 0/100/0/ approx. 50 5 100 —  85 15 — 322.4 65  1 95 4.6 50/50/0/ approx. 20 Outer layer thick- Particles in (A)Film nesses SPAN Film thickness (A) (C) Diameter 98 Concentrationd₅₀/d_((A)) structure μm μm μm — % by wt. ratio Examples 1 ABC 25 3 1.15.5 1.8 4.50 1.83 2 ABC 25 3 1.1 5.5 1.8 4.00 1.83 3 ABC 25 3 1.1 5.51.8 3.00 1.83 4 ABC 25 3 1.1 5.5 1.8 4.00 1.83 Comparative 1 ABC 25 3 15.5 1.8 5 1.83 Examples 2 ABC 70 2 — — — — 3 AB 20 2.98 — 1.5 + 5 — 0.31.68 4 AB 17.2 4.1 — — — — — 5 AB 11.5 2.5 — 2   — 0.25 0.8 TAterephthalate, IA isophthalate, EG ethylene, BD butane, NGneopentylglycol AzA azelate, SeA sebacate, AdA adipate, FA furtherdicarboxylic acids and glycols

TABLE 3 Minimum Seal seam strength sealing with respect to trays Peeltest (= Roughness R_(a) temperature 160° C. 180° C. 200° C. peelingShrinkage in Side A Side C Tray ° C. N/15 mm performance) Opacity % TD %nm Examples 1 CPET 152 1.7 2.4 4.4 ++++ 15 8 225 60 2 CPET 150 2.1 3.87.1 ++++ 12 8 220 60 3 CPET 149 1.5 2.6 5.1 ++++ 11 8 210 60 4 CPET 1384.8 4.7 6.1 ++++ 13 10 222 60 C-Examples 1 CPET 105 3.5 5.0 8.0 − 12 —310 60 2 CPET 109 4.5 5.0 6.5 ++ 10 50 35 60 3 CPET 109 4.2 5.5 8.1 − —— 69 25 4 CPET 112 2.0 4.0 6.0 − — — 33 20 5 CPET 110 3.0 4.0 5.0 − — —120 22 Peel test: ++++: At all sealing temperatures, film is peeled fromthe tray without the film starting or continuing to tear. Impeccable,smooth, clean peeling of the film from the tray, even in the uppertemperature range at high seal seam strength. −: At all sealingtemperatures, film starts to tear on removal from the tray. ++: Between++++ and −

1. A coextruded, biaxially oriented polyester film which has a baselayer (B) and has a coextruded, uncoated heatsealable outer layer (A)that can be peeled from APET/CPET and from CPET, where the outer layer(A) comprises a) from 80 to 98% by weight of polyester and b) from 2 to10% by weight of inorganic and/or organic particles with a mediandiameter d₅₀ of from 3 to 12 μm, and where c) the polyester used to formsaid layer A is composed of from 12 to 89 mol % of units derived from atleast one aromatic dicarboxylic acid and of from 11 to 88 mol % of unitsderived from at least one aliphatic dicarboxylic acid, where the totalof the dicarboxylic-acid-derived molar percentages is 100, d) the ratiocalculated from the particle size d₅₀ of the particles and the layerthickness d_(A) of the outer layer (A) is greater than or equal to 1.5,e) the shrinkage of the film, at least in one direction, is more than 5%measured at 100° C. over a period of 15 min and f) said film does nottear during removal from CPET, based on a sealing temperature of 160° C.and g) said top layer (A) exhibits a surface roughness, Ra, ranging fromgreater than 120 to 400 nm, as determined via DIN
 4768. 2. The polyesterfilm as claimed in claim 1, wherein the thickness of the outer layer (A)d_(A) is from 1.0 to 8 μm.
 3. The polyester film as claimed in claim 1,wherein the aromatic dicarboxylic acids have been selected from one ormore of the following substances: terephthalic acid, isophthalic acid,and 2,6-naphthalenedicarboxylic acid.
 4. The polyester film as claimedin claim 1, wherein the aliphatic dicarboxylic acids have been selectedfrom one or more of the following substances: succinic acid, pimelicacid, suberic acid, azelaic acid, sebacic acid, glutaric acid, andadipic acid.
 5. The polyester film as claimed in claim 1, wherein thepolyester of the outer layer (A) contains from 12 to 89 mol % ofterephthalate, from 0 to 25 mol % of isophthalate, from 11 to 88 mol %of azelate, from 0 to 50 mol % of sebacate, from 0 to 50 mol % ofadipate, and more than 30 mol % of ethylene or butylene, based in eachcase on total amount of dicarboxylate and, respectively, total amount ofalkylene.
 6. The polyester film as claimed in claim 1, wherein the outerlayer (A) has a minimum sealing temperature of not more than 165° C. forsealing against APET/CPET or CPET trays.
 7. The polyester film asclaimed in claim 1, wherein the outer layer (A) has a seal seam strengthof at least 1 N/15 mm of film width against APET/CPET or CPET trays. 8.The polyester film as claimed in claim 1, which has three layers and hasan A-B-C structure.
 9. The polyester film as claimed in claim 1, whereinthe outer layer (C) also comprises inorganic or organic particles. 10.The polyester film as claimed in claim 1, wherein the particles in theouter layer (A) have a SPAN98 of ≦2.0.
 11. The polyester film as claimedin claim 1, wherein the base layer (B) is composed of at least 80% byweight of thermoplastic polyester.
 12. The polyester film as claimed inclaim 1, wherein the polyester of the base layer (B) containsterephthalate units and/or isophthalate units, and ethylene units.
 13. Aprocess for producing a polyester film as claimed in claim 1,encompassing the steps of a) producing a multilayer film via coextrusionand shaping of the melts to give flat melt films, b) biaxial stretchingof the film, and c) heat-setting of the stretched film.
 14. The processas claimed in claim 13, wherein, to increase the shrinkage of the film,the setting time and the setting temperature during the heat-settingprocess are lowered.
 15. A lid film for APET/CPET or CPET trayscomprising polyester film as claimed in claim
 1. 16. A film according toclaim 1, wherein the shrinkage of the film is increased within thetransverse direction alone and ranges from more than 5% to 30%, measuredat 100° C. over a period of 15 min or the shrinkage of the film isincreased in both the transverse and machine directions and ranges frommore than 5 to 20%, measured at 100°0 C. over a period of 15 min. 17.APET/CPET or CPET trays comprising lid film which further comprisespolyester film as claimed in claim 1, wherein said trays areself-venting.
 18. A film according to claim 1, wherein a portion of thepolyester used to form said layer (A) exhibits a glass transitiontemperature, Tg, of more than 50° C.
 19. A film according to claim 1,wherein said base layer (B) is formed from polymer consisting entirelyof one or more of polyethylene terephthalate; polyethylene2,6-naplithalate; poly- 1,4-cyolohexanedimethylene terephthalate;polyethylene 2,6-naphthalate bibenzoate and copolymers thereof.
 20. Acoextruded, biaxially oriented polyester film which has a base layer (B)and a heatsealable outer layer (A) that can be peeled from APET/CPET andfrom CPET, where the outer layer (A) comprises a) from 80 to 98% byweight of polyester and b) from 2 to 10% by weight of inorganic and/ororganic particles with a median diameter d₅₀ of from 3 to 12 μm, andwherein c) the polyester of the outer layer (A) is prepared from twopolyesters I and II, said polyester comprising from 12 to 89mol % ofunits derived from at least one aromatic dicarboxylic acid and of from11 to 88 mol % of units derived from at least one aliphatic dicarboxylicacid, where the total of the dicarboxylic-acid-derived molar percentagesis 100, and d) the ratio calculated from the particle size d₅₀ of theparticles and the layer thickness d_(A) of the outer layer (A) isgreater than or equal to 1.5, e) the shrinkage of the film, at least inone direction, is more than 5%, measured at 100° C. over a period of 15mm and g) said top layer (A) exhibits a surface roughness, Ra, rangingfrom greater than 120 to 400 nm, as determined via DIN
 4768. 21. Thepolyester film as claimed in claim 20, wherein the polyester I iscomposed of one or more aromatic dicarboxylates and of one or morealiphatic alkylenes.
 22. The polyester film as claimed in claim 20,wherein the polyester I contains terephthalate units, isophthalateunits, and ethylene units.
 23. The polyester film as claimed in claim20, wherein the proportion of the polyester I in the outer layer (A) isfrom 0 to 50% by weight.
 24. The polyester film as claimed in claim 20,wherein the polyester I has a glass transition temperature above 50° C.25. The polyester film as claimed in claim 20, wherein the polyester IIis composed of one or more aliphatic dicarboxylates and of one or morearomatic dicarboxylates, and of one or more aliphatic alkylenes.
 26. Thepolyester film as claimed in claim 20, wherein the polyester II containsazelate units, terephthalate units, isophthalate units, and ethyleneunits.
 27. The polyester film as claimed in claim 20, wherein theproportion of the polyester II in the outer layer (A) is from 50 to 100%by weight.
 28. The polyester film as claimed in claim 20, wherein thepolyester H has a glass transition temperature below 20° C.